Heartworms are parasites that live in the arteries of the lungs and right side of the heart in dogs and wild canines. They can live up to seven years and eventually cause right-heart failure and death.

    The official name for heartworms is dirofilarial immitis. They are found in most parts of the world and in every state of the United States. Heartworms also occur in cats, ferrets and California sea lions. Heartworms are spread from an infected animal to other animals by mosquitos. Heartworm disease is only spread by the bite of mosquitoes that contain immature heartworms (larvae) in their bodies.

    Once a dog is bitten by an infected mosquito, the larvae travel to the lungs, where they develop into adult worms. The adult worms cause damage to the pulmonary arteries and lining of the heart.

    Most dogs with early heartworm disease have no clinical signs, and the disease is detected by routine blood tests by your veterinarian. As the infestation of worms worsens, clinical signs include lethargy, decreased activity and especially coughing. As the disease worsens, breathing rate and effort increase. With advanced disease and right-heart failure, fluid builds up in the abdomen, and weight loss occurs.

    If a dog tests positive on blood tests for heartworm, chest X-rays are done to look for heart and lung disease caused by the worms. Often, an ultrasound of the heart is also performed to evaluate heart function. Routine blood tests are recommended to evaluate liver and kidney function, because the drug used to kill adult heartworms can adversely affect the kidneys and liver.

    Adult heartworms are killed with a drug called melarsomine. Two doses of melarsomine are given by injection 24 hours apart. This regimen kills 90-95 percent of adult heartworms within two to three weeks. Ivermectin, selamectin or milbemycin are drugs used to kill the circulating immature heartworms (larvae).

    Once the adult heartworms and circulating larvae are treated, heartworm preventative drugs are started. These prevent adult heartworms from developing after a dog is bitten by an infected mosquito.

    Dogs with early and mild heartworm infections have a good prognosis after treatment. Dogs with older infections that have already damaged the heart and lungs have a more guarded prognosis for recovery and long-term health.

    If you are taking your pets to an area of the country with a lot of mosquitoes, talk to your veterinarian about heartworm prevention, as an ounce of prevention is worth a pound of cure.


Dr. Karsten Fostvedt is a veterinarian at St. Francis Pet Clinic in Ketchum.

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